1. Field of Invention
This invention relates to induced current linear and rotary absolute position transducers. In particular, this invention is directed to rotary and linear induced current absolute position transducers with improved winding configurations to increase the proportion of the usefull output signal component, i.e. those related to transducer position, relative to extraneous ("offset") components of the output signal which are unrelated to transducer position.
2. Description of Related Art
Induced current position transducers typically have a read head that is movable relative to a scale member. The position transducer may have a read head with one or more transducers each having a transmitter winding and overlapping receiver windings. Each transducer will have a scale on the scale member. Each scale on the scale member includes a plurality of flux modulators. Importantly, the receiver windings of each transducer have a wavelength which is different from other receiver windings. Similarly, the flux modulators each have a length measured along the measuring axis, equal to one-half of the wavelength of the corresponding receiver winding.
Each transducer uses two overlapping receiver windings which are spaced one-quarter of a scale wavelength apart to provide output signals which are in quadrature. The signals output from the receiver windings are, therefore, 90 degrees out of phase with each other. The relation between the signals from the two receiver windings allows the direction of movement to be determined.
The signal amplitudes of the receiver windings follow a sinusoidal function as the scale moves along the measuring axis. Each of the receiver windings have loops with alternating winding directions. The receiver windings each have a first set of loops with a positive polarity and a second set of loops with a negative polarity which are individually positioned between each adjacent loop in the first set. Thus, the electromotive force (EMF) induced in the positive polarity loops has a polarity that is opposite to the polarity of the EMF induced in the negative polarity loops. The positive polarity loops generally enclose the same size area as the negative polarity loops and, thus, nominally the same amount of magnetic flux. Therefore, the absolute magnitude of the EMF generated in the positive polarity loops is nominally the same as the EMF generated in the negative polarity loops.
The number of positive polarity loops are also equal to the number of negative polarity loops. Thus, the positive polarity of EMF induced in the positive polarity loops is exactly offset by the negative polarity EMF induced in the negative polarity loops. Accordingly, the net nominal EMF on each of the receiver windings is zero and it is intended that no signal is output from the receiver windings as a result of the direct coupling from the transmitter windings to the receiver windings.
When the read head is placed in proximity to the scale, the changing magnetic flux generated by the transmitter winding also passes through the flux modulators. The flux modulators modulate the changing magnetic flux.
FIG. 1A shows the position-dependent output from the-positive polarity loops as the flux modulators move relative to the positive polarity loops. Assuming the flux modulators are flux disrupters, the minimum signal amplitude corresponds to those positions where the flux disrupters exactly align with the positive polarity loops, while the maximum amplitude positions correspond to the flux disrupters being aligned with the negative polarity loops.
FIG. 1B shows the signal output from each of the negative polarity loops. As with the signal shown in FIG. 1A, the minimum signal amplitude corresponds to those positions where the flux disrupters exactly align with the positive polarity loops, while the maximum signal output corresponds to those positions where the flux disrupters exactly align with the negative polarity loops. It should be appreciated that if flux enhancers were used in place of flux disrupters, the minimum signal amplitudes in FIGS. 1A and 1B would correspond to the flux enhancers aligning with the negative polarity loops, while the maximum signal amplitude would correspond to the flux enhancers aligning with the positive polarity loops.
FIG. 1C shows the net signal output from either of the overlapping receiver windings. This net signal is equal to the sum of the signals output from the positive and negative polarity loops, i.e., the sum of the signals shown in FIGS. 1A and 1B. The net signal show in FIG. 1C should ideally be symmetrical around zero, that is, the positive and negative polarity loops should be exactly balanced to produce a symmetrical output with zero offset.
However, a "DC" (position independent) component often appears in the net signal in a real device. This DC component is the offset signal V.sub.o. This offset V.sub.o is an extraneous signal component which complicates signal processing and leads to undesirable position measurement errors. This offset has two major sources.
First, the full amplitude of the transmitter fields pass through the receiver windings. As outlined above, this induces a voltage in each loop. The induced voltage is nominally canceled because the loops have opposite winding directions. However, to perfectly cancel the induced voltage in the receiver windings requires the positive and negative loops to be perfectly positioned and shaped, for a perfectly balanced result. The tolerances on the balance are critical because the voltages induced directly into the receiver winding loops by the transmitter windings are much stronger than the modulation in the induced voltage caused by the flux modulators. In practice, fabrication tolerances always prevent perfect balance.
Second, the spatially modulated field created by the flux modulators also exhibits an average position-independent offset component. That is, the flux modulators within the magnetic field generated by the transmitter windings all create the same polarity spatial modulation in the magnetic field. For example, when flux disrupters are used, the induced eddy current field from the flux modulators has an offset because the flux disrupters within the transmitter fields all create a same polarity secondary magnetic field. At the same time, the space between the flux disrupters does not create a secondary magnitude field.
Thus, each positive polarity loop and each negative polarity loop of the receiver windings sees a net magnetic field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function is not balanced around zero, i.e., it has a large nominal offset. Similarly, when flux enhancers are used, the field modulation due to the flux enhancers has a bias because the enhancers within the transmitter windings all create the same field modulation, while the space between the modulators provides no modulation. Each positive and negative polarity loop of each receiver winding, therefore, sees a spatially modulated field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function also has a large nominal offset.
A receiver winding having an equal number of similar positive and negative polarity loops helps eliminate the offset components. However, any imperfection in the balance between the positive and negative polarity loops allows residual offsets according to the previous description.
Both of these offset components are expected to be canceled solely by the symmetry between the positive and negative polarity loops in the receiver windings. This puts very stringent requirements on the manufacturing precision of the receiver windings. Experience in manufacturing the transducer indicates it is practically impossible to eliminate this source of error from an induced current position transducer.